Damp proofing garage

Hi, First post here. Im nearly finished my garage to studio design submission for this forum to consider but i have done it very detailed (as required) and so thought i would ask about damp proofing existing structure as a separate issue beforehand. The garage has a 4-5" cement floor with NO plastic sheet built in DPC, walls are single skin brick. I have already excavated to foundations (on outerside of brick walls) and put in a DPC up to ground level as rain water was flooding the garage floor. This has kept floor dry. Everything stored in garage is wet - even things off the floor, so the humidity in there must be high as there is mould on plasterboard ceiling (only put up a few months ago). I should point out im in Durham in England where its always damp. I contacted a damp proofing expert for advice (he was selling expensive specialised products) and he said if i can get a dehumidifier (with a humidity meter) and make room as air tight as possible and get room temperature up (i have a large radiator in room) then it should be ok once covered floor and walls in a non permeable substance. But i just cant see being able to get room hot enough to hold the moisture in the air with it being single skin brick.(ceiling has 100mm rockwool under 12.5 mm gypsum board sealed with caulk.) I did the plastic sheet on floor test and it was wet the next day so dampness is coming through the floor, the walls also get soaked after heavy rain in certain areas but do dry out within a few days. I intend to get a dehumidifier (with a humidistat) and keep the large radiator ive had fitted on the same temperature as in the house and check for any cracks in the walls brick/mortar, but i would like advice specifically about what to use to seal the floor and to stop moisture coming through walls (but the main problem is definately the floor). Would bitumen or a paintable sealant work on the floor and walls (underlay and carpet will be going ontop of floor so it wont get damaged). When i searched here i found a thread which said "Do not go with products like drylok or other 'paint-on' sealant products, they all will eventually separate and crack and need re-applying". It also mentioned Xypex. Could anyone give me advice as to the best product (on a small budget) to use on the floor and walls as once the conversion is done, if dampness does reoccur (due to product cracking) it would be big trouble. I would really appreciate your help as i also have to finish mixing some tracks in here before the conversion (had to give up the previously rented studio space), but theres no way i can put gear into this kind of dampness. Cheers Peter

edit you profile so we know were you are. Sound like a big job! What and see what the builders on the forum say

Hi, Ive sent john an email as i cant edit profile when i try. As said above - i live in Durham uk. As an update, ive found room does actually have a DPC down and its just walls that are problem, so should be able to sort that. My main problem is going to be getting fresh air in and out of the room as baffled ventilation systems seem very bulky (space which i can hardly afford, and i cant alter appearance of garage outside as its a conservation area) and expensive (again which i cant afford). My only option here it seems is to get a dehumidifier in and open the door regularly for air. This will be a right pain, so does anyone know of any miracle work arounds! Peter

Only what was suggested at recording.org.